For those requiring clean liquids, e.g., water, oil and the like, the need to filter particulate matter from such liquids has long been an area of concern. And workers in the field of filter engineering have responded to those concerns by providing efficient filter devices configured to remove particulates, the size(s) of which are consistent with user needs.
Commonly, liquid filters use a porous filtering medium mounted on or held in place by some type of supporting structure. An earlier, highly-effective filter module used primarily for swimming pool applications is disclosed in U.S. Pat. No. 3,988,244 (Brooks) and a much more recent innovation in pool filters is disclosed in U.S. Pat. No. 5,190,651 (Spencer). The filter modules disclosed in the Brooks and Spencer patents (owned by Sta-Rite Industries, Inc., the leading manufacturer of pool and spa filters) include two pleated filter media mounted on a lattice-like core structure. The media and the core structure are securely bonded to one another by end caps molded thereto.
The matter of media-core bonding has been the subject of a good deal of creative effort. One approach to bonding filter elements is disclosed in U.S. Pat. No. 3,042,571 (Jackson). The method described in such patent involves placing a bead of adhesive along each of the two parallel edges of a sheet-like material from which a pleated medium is formed after adhesive application. After the medium is wrapped to cylindrical shape, its ends (which have the adhesive beads thereon) are pressed against the end caps for bonding.
A second method of bonding is described in U.S. Pat. No. 3,392,843 (Mumby) and involves a cylindrical filter cartridge having a wrapped, pleated filter medium. The method includes placing several beads of adhesive on a filter medium which is pleated but not yet wrapped around the support structure. Adhesive placement is along spaced-apart "lines" perpendicular to the edges of the pleats. After the adhesive hardens, the medium is wrapped and placed into a perforated cylindrical enclosure. The beads function to retain the medium in a slightly-spaced relationship to such enclosure.
U.S. Pat. No. 4,120,711 (Gudeman) described equipment and a process for sealing molded end caps to a filter tube by dispensing a predetermined quantity of molten adhesive into an end cap. Thereafter, a cylindrical filter cartridge is inserted into the end cap under slight pressure to embed the ends of the filter in the adhesive.
A similar approach involves using an open mold into which is placed a liquid plastisol material or molten polyurethane. Pleated media are then inserted into such material which is allowed to harden. The end caps (or "retaining plates" as they are referred to in the Brooks patent) are thereby cast in place. This approach has been used to make the filter cartridges described in the Brooks and Spencer patents.
While there is a good deal of patent literature on filter devices and how to make them, there seems to be much less information available on equipment that uses bonding techniques, especially equipment using such techniques to make filter devices--the Gudeman patent is an exception.
Another exception is U.S. Pat. No. 4,925,601 (Vogt et al.) which describes equipment used to apply viscous thermoplastic polymer to what the patent describes as a melt-blown web. The web is flat and strip-like, moves in a linear path, and the polymer is applied through elongate nozzles above the web and perpendicular to the direction of web travel. The resulting medium is said to be useful for filtering lubricating coolants.
U.S. Pat. No. 4,256,526 (McDaniel) discloses equipment that uses adhesive bonding but for making paper bags and corrugated cartons, not filters. Carton stock moves in a linear path and beads of adhesive are placed along lines parallel to the direction of stock movement. At the proper instant, adhesive is placed along a line perpendicular to such direction by using a burst from an elongate nozzle.
While known methods and equipment for making filters have been generally satisfactory, problems are not unknown. For example, urethane is relatively expensive and because of between-operation purging of urethane guns, a good deal of the product is wasted. Another difficulty attending use of liquid urethane is that precise "mix" formula is difficult to control. And urethane cures with very sharp edges which makes it necessary to hand-trim flashing and the like. In other words, filter-making processes using cast urethane tend to be somewhat wasteful and are certainly labor-intensive.
Another problem with known technology is that irrespective of whether plastisol or urethane is used, only one filter end cap can be made at a time. Urethane has about a 5 minute cure time for each end cap and neither the filter medium nor the end cap can be moved until hardening occurs.
An improved method and apparatus for making filter modules in a way which dramatically shortens manufacturing time, which uses less expensive materials and which substantially avoids the need for hand-trimming would be an important advancement in the art.